My MSIS degree is in Human Computer Integration. The foundation of this is to NOT alter a function that users are accustomed to, but add new features in such a way that folks are not frustrated with using the application. Every time MS moves a feature users use all the time to a new location, or alter a feature so drastically that it no longer resembles the user work habits, they are no longer adding value but creating frustration.
The success of a product means that each new version or iteration does not create a huge learning curve for the users, but extends the new features without altering historical work habits.
Like Ben said about the 'pizza for 400 million', new features should meld with the old, and not totally re-write how things function with each new version.